Phase-change materials (PCMs) are materials that are capable of storing or releasing energy in a latent manner during a reversible physical change of state. For example, such materials can store thermal energy if, in their liquid state, they are exposed to temperatures below their melting point, thereby inducing them to solidify and release an amount of energy corresponding to her heat of fusion. They can store thermal energy when this process is carried out in reverse. Such materials are useful because they can release and store much more thermal energy that typical insulating materials of comparable volume.
PCMs are all the more advantageous when the amount of heat exchanged during these phase changes is large. In the case of a solid/liquid transition, this is expressed by their heat of fusion or the heat of crystallization. The melting temperatures and crystallization temperatures of the PCMs furthermore determine the possible applications of the material. Typical uses include cooling food products or of pharmaceutical products that are sensitive to heat, cooling textile materials, cooling engines, cooling electronic components and circuits, or cooling waste combustion plants. On the industrial scale, PCMs also form a means for recovering heat that is released or that is available contained in containers or equipment (such as chemical reactors, generators of electrical or mechanical energy) or in process streams or fluids (for example cooling/heating circuits, effluents, etc.), especially during certain exothermic chemical reactions. The heat released may then be reused to provide energy to other reactions, which makes it possible to reduce the industrial consumption of fossil fuel-derived energy or electrical energy. The use of PCMs in air conditioning cycles (heating/cooling) is also an example of an application where it is possible to efficiently store thermal energy to then release it at the desired time and in the desired amount.
The most common PCM material is water. During its solid/liquid transition (and therefore implicitly liquid/solid transition), it makes it possible to absorb or release large amounts of heat. It is in this way that tanks or pools of water at temperatures near 0° C., which is free (in the form of an ice/liquid water mixture) or is encapsulated, for example in balls made of plastic or other materials, constitute a simple example of a PCM system capable of storing (during the melting of the ice) and of releasing (during the crystallization of the liquid water) large amounts of energy per unit mass of water. But water, although readily available and non-toxic, cannot solve all the possible problems due to its restricted usage range (around 0° C.) and problems linked to the high volume expansion of the ice during its formation.
Other common PCMs include paraffins and fatty acids, such as dodecanoic acid, hydrated salts such as manganese (II) nitrate dihydrate, certain eutectic mixtures, such as mixtures of capric acid and of lauric acid. These compounds generally have quite low melting temperatures, ranging from 15 to 48° C. Even so, these PCMs have many drawbacks impeding their industrial development. In particular, organic PCMs may be inflammable, have a low thermal conductivity in the solid state, require high heat transfers during the freezing cycle, and have a low volumetric latent heat. Furthermore, paraffins may pose problems of supply, cost and generation of CO2 due to their petroleum origin. Inorganic PCMs generate, for their part, significant supercooling phenomena. Moreover, their phase transition temperatures are not constant due, in particular, to their hygroscopicity. Finally, they are capable of resulting in a corrosion of the metals with which they are in contact.
Therefore, there is a continuing need to develop additional PCMs that have a high heat of fusion, have melting points that occur at practical temperatures for various applications, and overcome other of the aforementioned problems.